Some hints:
- Viking Line is normally cheaper than Silja
- cruises are often cheaper than one way, you may consider booking a cruise and disregarding the return
- Friday is the most expensive departure day
- If choosing Silja, join Club ONE and get a minimum 10% discount
- booking meals in advance saves you $$
Back to your questions, yes, A-class is a nice outside cabin (with sea views) and probably the class I would recommend to someone not travelling this route often. Of course Commodore class would be the choice of a normal flyertalker
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I can't imagine not using the restaurant services on board, to me that is an integral part of the experience. I have had so many buffets on board, though, so I always consider a la carte as an option.
There is nothing fun about flying Stockholm - Helsinki. Besides, taking the boat saves you one night accomodation and expensive airport transfers.